The present invention relates to a process for increasing the contents of mesophase in a pitch. More particularly it relates to such a process in which a high molecular aromatic fraction based on coal tar and/or crude oil in molten form and in form of a thin, continuously moving film is thermally treated, and volatile components are removed during the thermal treatment by degassing.
The process of this type is known in the art. Such a process for treatment of heavy crude oil pitch is disclosed for example in the German document DE-OS No. 2,925,549.
Carbon fibers and filaments have a great importance, for example as reinforcing fibers in synthetic plastic materials. They can be produced for example from polyacrylnitrile fibers or from pitch precursor fibers.
Since in many applications of these carbon fibers high strength and modulus values are required, many researches have been conducted to increase these values. It was found that the strength and modulus value of the carbon fibers can be increased by suitable selection and pretreatment of the initial material. It has been known for a long time that the pitch fibers with a high contents of mesophase are very well suitable for subsequent treatment to carbon fibers with high strength and high Young-modulus. This subsequent treatment can be performed by known methods and generally includes a pre-oxidation, and subsequent carbonization and in some cases graphitization.
Pitch fibers with high mesophase contents are produced by melt spinning of a pitch which also has a high mesophase contents. Mesophase is interpreted as regions which contain the aggregation of oriented molecule units having flow condition. Since the reorientation is formed substantially in a predetermined direction, mesophase regions have optical anisotropy. For this reason the mesophase in pitch is identified also as anisotropic phase or as anisotropic region. The contents of a pitch as to mesophase can be determined for example by polarized light, for example with a polarization microscope. Also, the solubility properties of the pitch in certain solvents can be used for determining the contents of mesophase, since the mesophase generally is considerably less soluble than the non-oriented (isotropic) fractions of the pitch.
Since pitches of conventional type, for example those recovered in conventional processes from coal tar pitch or crude oil pitch are isotropic without preceeding thermal treatment, their contents as to mesophase must be increased by respective pre-treatment if pitch fibers with high mesophase contents must be produced by melt spinning. Processes were developed to increase the mesophase contents of a pitch. It was found that by a temperature treatment with inert gas, for example, in the region of between 250.degree. and 500.degree. C., the mesophase contents is increased, which can be traced back to chemical processes such as polymerization and physical processes such as evaporation of low molecular, readily volatile isotropic components.
When it is necessary with such known thermal processes as heating of the pitch in an open vessel, to convert isotropic solid-free pitches into pitches with high mesophase contents, for example, in pitch with 80-100% mesophase, many disadvantages and difficulties occur. On the one hand, such processes consume a lot of time, since the production of the mesophase and the volatilization of the low boiling components from the interior of the pitch runs very slow. Increase of the treatment temperature accelerates the production of the mesophase; however, the increased quantity of solid components, such as coke, which are not suitable for the melt spinning is formed at the increased temperature. Moreover, softening temperatures, flow temperatures, viscosity of the mesophase fraction increase with increasing treatment time and treatment temperature. For the melt spinning of mesophase pitch, very high flow temperatures and viscosities are however not desirable.
Attempts were made to increase the mesophase contents of pitch within shorter times. One possibility is described in U.S. Pat. No. 3,974,264. In the process disclosed in this reference an inert gas is supplied during the thermal treatment through the pitch for removing the readily voltatile components faster. The disadvantage of this process is that here also relatively long treatment times are needed, since it is performed in conventional vessels with high pitch volumes. In this process there is also the danger of multiplied coke formation and increased viscosity of the mesophase fractions, since the formed mesophase regions further polymerize when the conversion of isotrope regions into mesophase is performed.
An improved process is disclosed in the German document DE-OS No. 2,925,549. In this process the temperature treatment of the pitch is performed so that pitch in form of a laminar stream is introduced into a reduced pressure vessel, sinks there downwardly under the action of the gravity force, and removed from the bottom of the vessel. A temperature up to 300.degree. C. is used in this process. The advantage of this process is that the pitch can be thermally treated as a thin film in a continuous fashion. Thereby volatile components can be removed faster than in the event of high pitch volumes during stationary treatment.
This process however possesses several disadvantages. First of all, the speed of the mesophase enrichment is relatively low because of the low treatment temperature. The low treatment temperature has a further disadvantage in that the increase of the contents of mesophase practically deals exclusively with removal of low molecular, readily volatile components, while no formation of mesophase because of polymerization occurs. The high molecular isotropic regions remain and are not converted to mesophase, so that pitch with very high mesophase contents cannot be produced in this process. Moreover, the pitch flows in form of a laminar stream without a mechanical homogenization. Since isotropic and mesophase fractions of the pitch are soluble in one another only to a limited extent, two phases of different viscosity can be produced in this process. Therefore a subsequent melt spinning is undersirably affected. Due to the fact that the mesophase has a higher specific weight than isotropic phase, it is also possible that during flowing of the pitch under the action of gravity force from above downwardly, differences in flow speed of the mesophase fraction relative to the isotropic fraction take place.